Apollo Guidance Computer

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Apollo Guidance Computer
Agc view.jpg
Apollo Guidance Computer and DSKY
Invented by Charles Stark Draper Laboratory
Manufacturer Raytheon
IntroducedAugust 1966;53 years ago (1966-08)
DiscontinuedJuly 1975;44 years ago (1975-07)
Guidance Computer
ProcessorDiscrete silicon integrated circuit (IC) chips (RTL based)
Frequency2.048 MHz
Memory15-bit wordlength + 1-bit parity,
2048 words RAM (magnetic-core memory), 36,864 words ROM (core rope memory) [1]
PortsDSKY, IMU, Hand Controller, Rendezvous Radar (CM), Landing Radar (LM), Telemetry Receiver, Engine Command, Reaction Control System
Power consumption55 W [2] :120
Weight70 lb (32 kg)
Dimensions24×12.5×6.5 inches(61×32×17 cm)

The Apollo Guidance Computer (AGC) is a digital computer produced for the Apollo program that was installed on board each Apollo command module (CM) and Apollo Lunar Module (LM). The AGC provided computation and electronic interfaces for guidance, navigation, and control of the spacecraft. [3]


The AGC has a 16-bit word length, with 15 data bits and one parity bit. Most of the software on the AGC is stored in a special read-only memory known as core rope memory, fashioned by weaving wires through and around magnetic cores, though a small amount of read/write core memory is available.

Astronauts communicated with the AGC using a numeric display and keyboard called the DSKY (for "display and keyboard", pronounced "DIS-kee"). The AGC and its DSKY user interface were developed in the early 1960s for the Apollo program by the MIT Instrumentation Laboratory and first flew in 1966. [4] The AGC was the first silicon integrated circuit based computer. The computer's performance was comparable to the first generation of home computers from the late 1970s, such as the Apple II, TRS-80, and Commodore PET. [5]


Astronauts manually flew Project Gemini with control sticks, but computers flew most of Project Apollo except briefly during lunar landings. [6] Each Moon flight carried two AGCs, one each in the command module and the lunar module, with the exception of Apollo 8 which did not need a lunar module for its lunar orbit mission. The AGC in the command module was the center of its guidance, navigation and control (GNC) system. The AGC in the lunar module ran its Apollo PGNCS (primary guidance, navigation and control system), with the acronym pronounced as pings.

The display and keyboard (DSKY) interface of the Apollo Guidance Computer mounted on the control panel of the command module, with the flight director attitude indicator (FDAI) above Dsky.jpg
The display and keyboard (DSKY) interface of the Apollo Guidance Computer mounted on the control panel of the command module, with the flight director attitude indicator (FDAI) above
Partial list of numeric codes for verbs and nouns in the Apollo Guidance Computer, printed for quick reference on a side panel Agc verb-noun-list.jpg
Partial list of numeric codes for verbs and nouns in the Apollo Guidance Computer, printed for quick reference on a side panel

Each lunar mission had two additional computers:


Flatpack silicon integrated circuits in the Apollo guidance computer Agc flatp.jpg
Flatpack silicon integrated circuits in the Apollo guidance computer

The AGC was designed at the MIT Instrumentation Laboratory under Charles Stark Draper, with hardware design led by Eldon C. Hall. [2] Early architectural work came from J.H. Laning Jr., Albert Hopkins, Richard Battin, Ramon Alonso, [7] [8] and Hugh Blair-Smith. [9] The flight hardware was fabricated by Raytheon, whose Herb Thaler [10] was also on the architectural team.

Following the use of integrated circuit (IC) chips in the Interplanetary Monitoring Platform (IMP) in 1963, IC technology was later adopted for the AGC. [11] The Apollo flight computer was the first computer to use silicon IC chips [12] (non-silicon integrated circuits may have earlier been used in a Texas Instruments computer).[ citation needed ] While the Block I version used 4,100 ICs, each containing a single three-input NOR gate, the later Block II version (used in the crewed flights) used 2,800 ICs, each with dual three-input NOR gates. [2] :34[ citation needed ] The ICs, from Fairchild Semiconductor, were implemented using resistor–transistor logic (RTL) in a flat-pack. They were connected via wire wrap, and the wiring was then embedded in cast epoxy plastic.[ citation needed ] The use of a single type of IC (the dual NOR3) throughout the AGC avoided problems that plagued another early IC computer design, the Minuteman II guidance computer, which used a mix of diode–transistor logic and diode logic gates.[ citation needed ] This is possible because NOR gates are universal logic gates from which any other gate can be made, though at the cost of using more gates. [13]

The computer had 2048 words of erasable magnetic-core memory and 36 kilowords of read-only core rope memory. Both had cycle times of 11.72 microseconds. The memory word length was 16 bits: 15 bits of data and one odd-parity bit. The CPU-internal 16-bit word format was 14 bits of data, one overflow bit, and one sign bit (ones' complement representation).[ citation needed ]

DSKY interface

Apollo computer DSKY user interface unit AGC user interface.jpg
Apollo computer DSKY user interface unit
LM DSKY interface diagram Apollo DSKY interface.svg
LM DSKY interface diagram

The user interface to the AGC was the DSKY, standing for display and keyboard and usually pronounced "DIS-kee". It has an array of indicator lights, numeric displays, and a calculator-style keyboard. Commands were entered numerically, as two-digit numbers: Verb, and Noun. Verb described the type of action to be performed and Noun specified which data were affected by the action specified by the Verb command.

Each digit was displayed via a green high-voltage electroluminescent seven-segment display; these were driven by electromechanical relays, limiting the update rate. Three five-digit signed numbers could also be displayed in octal or decimal, and were typically used to display vectors such as space craft attitude or a required velocity change (delta-V). Although data was stored internally in metric units, they were displayed as United States customary units. This calculator-style interface was the first of its kind.

The command module has two DSKYs connected to its AGC: one located on the main instrument panel and a second located in the lower equipment bay near a sextant used for aligning the inertial guidance platform. The lunar module had a single DSKY for its AGC. A flight director attitude indicator (FDAI), controlled by the AGC, was located above the DSKY on the commander's console and on the LM.


The AGC timing reference came from a 2.048 MHz crystal clock. The clock was divided by two to produce a four-phase 1.024 MHz clock which the AGC used to perform internal operations. The 1.024 MHz clock was also divided by two to produce a 512 kHz signal called the master frequency; this signal was used to synchronize external Apollo spacecraft systems.

The master frequency was further divided through a scaler , first by five using a ring counter to produce a 102.4 kHz signal. This was then divided by two through 17 successive stages called F1 (51.2 kHz) through F17 (0.78125 Hz). The F10 stage (100 Hz) was fed back into the AGC to increment the real-time clock and other involuntary counters using Pinc (discussed below). The F17 stage was used to intermittently run the AGC when it was operating in the standby mode.

Central registers

The AGC had four 16-bit registers for general computational use, called the central registers:

A:The accumulator, for general computation
Z:The program counter – the address of the next instruction to be executed
Q:The remainder from the DV instruction, and the return address after TC instructions
LP:The lower product after MP instructions

There were also four locations in core memory, at addresses 20-23, dubbed editing locations because whatever was stored there would emerge shifted or rotated by one bit position, except for one that shifted right seven bit positions, to extract one of the seven-bit interpretive op. codes that were packed two to a word. This was common to Block I and Block II AGCs.

Other registers

DSKY and AGC prototypes on display at the Computer History Museum. The AGC is opened up, showing its logic modules. ApolloGuidanceComputerwithDSKY.dwt.jpg
DSKY and AGC prototypes on display at the Computer History Museum. The AGC is opened up, showing its logic modules.
Prototype logic module from Block I AGC AGClogicmodule.agr.jpg
Prototype logic module from Block I AGC
Block II logic module, with flat-pack ICs Agc mount.jpg
Block II logic module, with flat-pack ICs
AGC dual 3-input NOR gate Agc nor2.jpg
AGC dual 3-input NOR gate
AGC dual 3-input NOR gate schematic Agc nor2a.jpg
AGC dual 3-input NOR gate schematic

The AGC had additional registers that were used internally in the course of operation:

S:12-bit memory address register, the lower portion of the memory address
Bank/Fbank:4-bit ROM bank register, to select the 1 kiloword ROM bank when addressing in the fixed-switchable mode
Ebank:3-bit RAM bank register, to select the 256-word RAM bank when addressing in the erasable-switchable mode
Sbank (super-bank):1-bit extension to Fbank, required because the last 4 kilowords of the 36-kiloword ROM was not reachable using Fbank alone
SQ:4-bit sequence register; the current instruction
G:16-bit memory buffer register, to hold data words moving to and from memory
X:The 'x' input to the adder (the adder was used to perform all 1's complement arithmetic) or the increment to the program counter (Z register)
Y:The other ('y') input to the adder
U:Not really a register, but the output of the adder (the 1's complement sum of the contents of registers X and Y)
B:General-purpose buffer register, also used to pre-fetch the next instruction. At the start of the next instruction, the upper bits of B (containing the next op. code) were copied to SQ, and the lower bits (the address) were copied to S.
C:Not a separate register, but the 1's complement of the B register
IN:Four 16-bit input registers
OUT:Five 16-bit output registers

Instruction set

The instruction format used 3 bits for opcode, and 12 bits for address. Block I had 11 instructions: TC, CCS, INDEX, XCH, CS, TS, AD, and MASK (basic), and SU, MP, and DV (extra). The first eight, called basic instructions, were directly accessed by the 3-bit op. code. The final three were denoted as extracode instructions because they were accessed by performing a special type of TC instruction (called EXTEND) immediately before the instruction.

The Block I AGC instructions consisted of the following:

TC (transfer control)
An unconditional branch to the address specified by the instruction. The return address was automatically stored in the Q register, so the TC instruction could be used for subroutine calls.
CCS (count, compare, and skip)
A complex conditional branch instruction. The A register was loaded with data retrieved from the address specified by the instruction. (Because the AGC uses ones' complement notation, there are two representations of zero. When all bits are set to zero, this is called plus zero. If all bits are set to one, this is called minus zero.) The diminished absolute value (DABS) of the data was then computed and stored in the A register. If the number was greater than zero, the DABS decrements the value by 1; if the number was negative, it is complemented before the decrement is applied—this is the absolute value. Diminished means "decremented but not below zero". Therefore, when the AGC performs the DABS function, positive numbers will head toward plus zero, and so will negative numbers but first revealing their negativity via the four-way skip below. The final step in CCS is a four-way skip, depending upon the data in register A before the DABS. If register A was greater than 0, CCS skips to the first instruction immediately after CCS. If register A contained plus zero, CCS skips to the second instruction after CCS. Less than zero causes a skip to the third instruction after CCS, and minus zero skips to the fourth instruction after CCS. The primary purpose of the count was to allow an ordinary loop, controlled by a positive counter, to end in a CCS and a TC to the beginning of the loop, equivalent to an IBM 360's BCT. The absolute value function was deemed important enough to be built into this instruction; when used for only this purpose, the sequence after the CCS was TC *+2, TC *+2, AD ONE. A curious side effect was the creation and use of CCS-holes when the value being tested was known to be never positive, which occurred more often than one might suppose. That left two whole words unoccupied, and a special committee was responsible for assigning data constants to these holes.
Add the data retrieved at the address specified by the instruction to the next instruction. INDEX can be used to add or subtract an index value to the base address specified by the operand of the instruction that follows INDEX. This method is used to implement arrays and table look-ups; since the addition was done on both whole words, it was also used to modify the op. code in a following (extracode) instruction, and on rare occasions both functions at once.
A special instance of INDEX (INDEX 25). This is the instruction used to return from interrupts. It causes execution to resume at the interrupted location.
XCH (exchange)
Exchange the contents of memory with the contents of the A register. If the specified memory address is in fixed (read-only) memory, the memory contents are not affected, and this instruction simply loads register A. If it is in erasable memory, overflow "correction" is achieved by storing the leftmost of the 16 bits in A as the sign bit in memory, but there is no exceptional behavior like that of TS.
CS (clear and subtract)
Load register A with the one's complement of the data referenced by the specified memory address.
TS (transfer to storage)
Store register A at the specified memory address. TS also detects, and corrects for, overflows in such a way as to propagate a carry for multi-precision add/subtract. If the result has no overflow (leftmost 2 bits of A the same), nothing special happens; if there is overflow (those 2 bits differ), the leftmost one goes the memory as the sign bit, register A is changed to +1 or −1 accordingly, and control skips to the second instruction following the TS. Whenever overflow is a possible but abnormal event, the TS was followed by a TC to the no-overflow logic; when it is a normal possibility (as in multi-precision add/subtract), the TS is followed by CAF ZERO (CAF = XCH to fixed memory) to complete the formation of the carry (+1, 0, or −1) into the next higher-precision word. Angles were kept in single precision, distances and velocities in double precision, and elapsed time in triple precision.
AD (add)
Add the contents of memory to register A and store the result in A. The 2 leftmost bits of A may be different (overflow state) before and/or after the AD. The fact that overflow is a state rather than an event forgives limited extents of overflow when adding more than two numbers, as long as none of the intermediate totals exceed twice the capacity of a word.
Perform a bit-wise (boolean) and of memory with register A and store the result in register A.
MP (multiply)
Multiply the contents of register A by the data at the referenced memory address and store the high-order product in register A and the low-order product in register LP. The parts of the product agree in sign.
DV (divide)
Divide the contents of register A by the data at the referenced memory address. Store the quotient in register A and the absolute value of the remainder in register Q. Unlike modern machines, fixed-point numbers were treated as fractions (notional decimal point just to right of the sign bit), so you could produce garbage if the divisor was not larger than the dividend; there was no protection against that situation. In the Block II AGC, a double-precision dividend started in A and L (the Block II LP), and the correctly signed remainder was delivered in L. That considerably simplified the subroutine for double precision division.
SU (subtract)
Subtract (one's complement) the data at the referenced memory address from the contents of register A and store the result in A.

Instructions were implemented in groups of 12 steps, called timing pulses. The timing pulses were named TP1 through TP12. Each set of 12 timing pulses was called an instruction subsequence. Simple instructions, such as TC, executed in a single subsequence of 12 pulses. More complex instructions required several subsequences. The multiply instruction (MP) used 8 subsequences: an initial one called MP0, followed by an MP1 subsequence which was repeated 6 times, and then terminated by an MP3 subsequence. This was reduced to 3 subsequences in Block II.

Each timing pulse in a subsequence could trigger up to 5 control pulses. The control pulses were the signals which did the actual work of the instruction, such as reading the contents of a register onto the bus, or writing data from the bus into a register.


AGC core rope memory (ROM) Agc rope.jpg
AGC core rope memory (ROM)

Block I AGC memory was organized into 1 kiloword banks. The lowest bank (bank 0) was erasable memory (RAM). All banks above bank 0 were fixed memory (ROM). Each AGC instruction had a 12-bit address field. The lower bits (1-10) addressed the memory inside each bank. Bits 11 and 12 selected the bank: 00 selected the erasable memory bank; 01 selected the lowest bank (bank 1) of fixed memory; 10 selected the next one (bank 2); and 11 selected the Bank register that could be used to select any bank above 2. Banks 1 and 2 were called fixed-fixed memory, because they were always available, regardless of the contents of the Bank register. Banks 3 and above were called fixed-switchable because the selected bank was determined by the bank register.

The Block I AGC initially had 12 kilowords of fixed memory, but this was later increased to 24 kilowords. Block II had 36 kilowords of fixed memory and 2 kilowords of erasable memory.

The AGC transferred data to and from memory through the G register in a process called the memory cycle. The memory cycle took 12 timing pulses (11.72 μs). The cycle began at timing pulse 1 (TP1) when the AGC loaded the memory address to be fetched into the S register. The memory hardware retrieved the data word from memory at the address specified by the S register. Words from erasable memory were deposited into the G register by timing pulse 6 (TP6); words from fixed memory were available by timing pulse 7. The retrieved memory word was then available in the G register for AGC access during timing pulses 7 through 10. After timing pulse 10, the data in the G register was written back to memory.

The AGC memory cycle occurred continuously during AGC operation. Instructions needing memory data had to access it during timing pulses 7-10. If the AGC changed the memory word in the G register, the changed word was written back to memory after timing pulse 10. In this way, data words cycled continuously from memory to the G register and then back again to memory.

The lower 15 bits of each memory word held AGC instructions or data, with each word being protected by a 16th odd parity bit. This bit was set to 1 or 0 by a parity generator circuit so a count of the 1s in each memory word would always produce an odd number. A parity checking circuit tested the parity bit during each memory cycle; if the bit didn't match the expected value, the memory word was assumed to be corrupted and a parity alarm panel light was illuminated.

Interrupts and involuntary counters

The AGC had five vectored interrupts:

The AGC responded to each interrupt by temporarily suspending the current program, executing a short interrupt service routine, and then resuming the interrupted program.

The AGC also had 20 involuntary counters. These were memory locations which functioned as up/down counters, or shift registers. The counters would increment, decrement, or shift in response to internal inputs. The increment (Pinc), decrement (Minc), or shift (Shinc) was handled by one subsequence of microinstructions inserted between any two regular instructions.

Interrupts could be triggered when the counters overflowed. The T3rupt and Dsrupt interrupts were produced when their counters, driven by a 100 Hz hardware clock, overflowed after executing many Pinc subsequences. The Uprupt interrupt was triggered after its counter, executing the Shinc subsequence, had shifted 16 bits of uplink data into the AGC.

Standby mode

The AGC had a power-saving mode controlled by a standby allowed switch. This mode turned off the AGC power, except for the 2.048 MHz clock and the scaler. The F17 signal from the scaler turned the AGC power and the AGC back on at 1.28 second intervals. In this mode, the AGC performed essential functions, checked the standby allowed switch, and, if still enabled, turned off the power and went back to sleep until the next F17 signal.

In the standby mode, the AGC slept most of the time; therefore it was not awake to perform the Pinc instruction needed to update the AGC's real time clock at 10 ms intervals. To compensate, one of the functions performed by the AGC each time it awoke in the standby mode was to update the real time clock by 1.28 seconds.

The standby mode was designed to reduce power by 5 to 10 W (from 70 W) during midcourse flight when the AGC was not needed. However, in practice, the AGC was left on during all phases of the mission and this feature was never used.

Data buses

The AGC had a 16-bit read bus and a 16-bit write bus. Data from central registers (A, Q, Z, or LP), or other internal registers could be gated onto the read bus with a control signal. The read bus connected to the write bus through a non-inverting buffer, so any data appearing on the read bus also appeared on the write bus. Other control signals could copy write bus data back into the registers.

Data transfers worked like this: To move the address of the next instruction from the B register to the S register, an RB (read B) control signal was issued; this caused the address to move from register B to the read bus, and then to the write bus. A WS (write S) control signal moved the address from the write bus into the S register.

Several registers could be read onto the read bus simultaneously. When this occurred, data from each register was inclusive-ORed onto the bus. This inclusive-OR feature was used to implement the Mask instruction, which was a logical AND operation. Because the AGC had no native ability to do a logical AND, but could do a logical OR through the bus and could complement (invert) data through the C register, De Morgan's theorem was used to implement the equivalent of a logical AND. This was accomplished by inverting both operands, performing a logical OR through the bus, and then inverting the result.


Margaret Hamilton standing next to listings of the software she and her MIT team produced for the Apollo Project. Margaret Hamilton - restoration.jpg
Margaret Hamilton standing next to listings of the software she and her MIT team produced for the Apollo Project.

AGC software was written in AGC assembly language and stored on rope memory. The bulk of the software was on read-only rope memory and thus couldn't be changed in operation, [15] but some key parts of the software were stored in standard read-write magnetic-core memory and could be overwritten by the astronauts using the DSKY interface, as was done on Apollo 14.

The design principles developed for the AGC by MIT Instrumentation Laboratory, directed in late 1960s by Charles Draper, became foundational to software engineering—particularly for the design of more reliable systems that relied on asynchronous software, priority scheduling, testing, and human-in-the-loop decision capability. [16] When the design requirements for the AGC were defined, necessary software and programming techniques did not exist so it had to be designed from scratch.

There was a simple real-time operating system designed by J. Halcombe Laning, [17] consisting of the Exec, a batch job-scheduling using cooperative multi-tasking [18] and an interrupt-driven pre-emptive scheduler called the Waitlist which could schedule multiple timer-driven 'tasks'. The tasks were short threads of execution which could reschedule themselves for re-execution on the Waitlist, or could kick off a longer operation by starting a "job" with the Exec.

The AGC also had a sophisticated software interpreter, developed by the MIT Instrumentation Laboratory, that implemented a virtual machine with more complex and capable pseudo-instructions than the native AGC. These instructions simplified the navigational programs. Interpreted code, which featured double precision trigonometric, scalar and vector arithmetic (16 and 24-bit), even an MXV (matrix × vector) instruction, could be mixed with native AGC code. While the execution time of the pseudo-instructions was increased (due to the need to interpret these instructions at runtime) the interpreter provided many more instructions than AGC natively supported and the memory requirements were much lower than in the case of adding these instructions to the AGC native language which would require additional memory built into the computer (at that time the memory capacity was very expensive). The average pseudo-instruction required about 24 ms to execute. The assembler and version control system, named YUL for an early prototype Christmas Computer, [19] enforced proper transitions between native and interpreted code.

A set of interrupt-driven user interface routines called Pinball provided keyboard and display services for the jobs and tasks running on the AGC. A rich set of user-accessible routines were provided to let the operator (astronaut) display the contents of various memory locations in octal or decimal in groups of 1, 2, or 3 registers at a time. Monitor routines were provided so the operator could initiate a task to periodically redisplay the contents of certain memory locations. Jobs could be initiated. The Pinball routines performed the (very rough) equivalent of the UNIX shell.

Many of the trajectory and guidance algorithms used were based on earlier work by Richard Battin. [17] The first command module flight was controlled by a software package called CORONA whose development was led by Alex Kosmala. Software for lunar missions consisted of COLOSSUS for the command module, whose development was led by Frederic Martin, and LUMINARY on the lunar module led by George Cherry. Details of these programs were implemented by a team under the direction of Margaret Hamilton. [20] Hamilton was very interested in how the astronauts would interact with the software and predicted the types of errors that could occur due to human error. [18] [20] In total, software development on the project comprised 1400 person-years of effort, with a peak workforce of 350 people. [17] In 2016, Hamilton received the Presidential Medal of Freedom for her role in creating the flight software.

The Apollo Guidance Computer software influenced the design of Skylab, Space Shuttle and early fly-by-wire fighter aircraft systems. [21] [22] The AGC code was uploaded to the internet in 2003, and the software itself was uploaded by a former NASA intern to GitHub on July 7, 2016. [23] [24] [25]

Block II

A Block II version of the AGC was designed in 1966. It retained the basic Block I architecture, but increased erasable memory from 1 to 2 kilowords. Fixed memory was expanded from 24 to 36 kilowords. Instructions were expanded from 11 to 34 and I/O channels were implemented to replace the I/O registers on Block I. The Block II version is the one that actually flew to the moon. Block I was used during the uncrewed Apollo 4 and 6 flights, and was on board the ill-fated Apollo 1.

The decision to expand the memory and instruction set for Block II, but to retain the Block I's restrictive three-bit op. code and 12-bit address had interesting design consequences. Various tricks were employed to squeeze in additional instructions, such as having special memory addresses which, when referenced, would implement a certain function. For instance, an INDEX to address 25 triggered the RESUME instruction to return from an interrupt. Likewise, INDEX 17 performed an INHINT instruction (inhibit interrupts), while INDEX 16 reenabled them (RELINT). Other instructions were implemented by preceding them with a special version of TC called EXTEND. The address spaces were extended by employing the Bank (fixed) and Ebank (erasable) registers, so the only memory of either type that could be addressed at any given time was the current bank, plus the small amount of fixed-fixed memory and the erasable memory. In addition, the bank register could address a maximum of 32 kilowords, so an Sbank (super-bank) register was required to access the last 4 kilowords. All across-bank subroutine calls had to be initiated from fixed-fixed memory through special functions to restore the original bank during the return: essentially a system of far pointers.

The Block II AGC also has the mysterious and poorly documented EDRUPT instruction (the name may be a contraction of Ed's Interrupt, after Ed Smally, the programmer who requested it) which is used a total of once in the Apollo software: in the Digital Autopilot of the lunar module. At this time, while the general operation of the instruction is understood, the precise details are still hazy, and it is believed to be responsible for problems emulating the LEM AGC Luminary software.[ citation needed ]

PGNCS trouble

DSKY and Buzz Aldrin on the Apollo 11 Lunar Module Eagle enroute to the Moon AS11-36-5391 (21710608621).jpg
DSKY and Buzz Aldrin on the Apollo 11 Lunar Module Eagle enroute to the Moon

PGNCS generated unanticipated warnings during Apollo 11's lunar descent, with the AGC showing a 1201 alarm ("Executive overflow - NO VAC AREAS"). There are five VAC areas of 44 (decimal) memory words each. If they were all consumed, a restart was initiated. [26] [ citation needed ]) and a 1202 alarm ("Executive overflow - NO CORE SETS"). [27] The cause was a rapid, steady stream of spurious cycle steals from the rendezvous radar (tracking the orbiting command module), intentionally left on standby during the descent in case it was needed for an abort. [28] [29]

During this part of the approach, the processor would normally be almost 85% loaded. The extra 6,400 cycle steals per second added the equivalent of 13% load, leaving just enough time for all scheduled tasks to run to completion. Five minutes into the descent, Buzz Aldrin gave the computer the command 1668, which instructed it to periodically calculate and display DELTAH (the difference between altitude sensed by the radar and the computed altitude). [nb 1] The 1668 added another 10% to the processor workload, causing executive overflow and a 1202 alarm. After being given the "GO" from Houston, Aldrin entered 1668 again and another 1202 alarm occurred. When reporting the second alarm, Aldrin added the comment "It appears to come up when we have a 1668 up". The AGC software had been designed with priority scheduling, and automatically recovered, deleting lower priority tasks including the 1668 display task, to complete its critical guidance and control tasks. Guidance controller Steve Bales and his support team that included Jack Garman issued several "GO" calls and the landing was successful. For his role, Bales received the US Presidential Medal of Freedom on behalf of the entire control center team and the three Apollo astronauts. [30]

The problem was not a programming error in the AGC, nor was it pilot error. It was a peripheral hardware design bug that had already been known and documented by Apollo 5 engineers. [31] However, because the problem had only occurred once during testing, they concluded that it was safer to fly with the existing hardware that they had already tested, than to fly with a newer but largely untested radar system. In the actual hardware, the position of the rendezvous radar was encoded with synchros excited by a different source of 800 Hz AC than the one used by the computer as a timing reference. The two 800 Hz sources were frequency locked but not phase locked, and the small random phase variations made it appear as though the antenna was rapidly "dithering" in position, even though it was completely stationary. These phantom movements generated the rapid series of cycle steals.

J. Halcombe Laning's software and computer design saved the Apollo 11 landing mission. Had it not been for Laning's design, the landing would have been aborted for lack of a stable guidance computer. [32] [33]

Applications outside Apollo

Fly By Wire testbed aircraft. The AGC DSKY is visible in the avionics bay Fly by wire.jpg
Fly By Wire testbed aircraft. The AGC DSKY is visible in the avionics bay

The AGC formed the basis of an experimental fly-by-wire (FBW) system installed into an F-8 Crusader to demonstrate the practicality of computer driven FBW. The AGC used in the first phase of the program was replaced with another machine in the second phase, and research done on the program led to the development of fly-by-wire systems for the Space Shuttle. The AGC also led, albeit indirectly, to the development of fly-by-wire systems for the generation of fighters that were being developed at the time. [34]

The AGC was also used for the United States Navy's Deep Submergence Rescue Vehicle. [35]

See also


  1. More specifically, verb 16 instructs the AGC to print the noun (in this case, 68, DELTAH) approximately twice per second. Had Aldrin known this, a simple 0668 (calculate and display DELTAH, once) would have only added approximately 5% load to the system, and would have only done so once, when ENTER was pressed.

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The CDC 6600 was the flagship of the 6000 series of mainframe computer systems manufactured by Control Data Corporation. Generally considered to be the first successful supercomputer, it outperformed the industry's prior recordholder, the IBM 7030 Stretch, by a factor of three. With performance of up to three megaFLOPS, the CDC 6600 was the world's fastest computer from 1964 to 1969, when it relinquished that status to its successor, the CDC 7600.

LINC Minicomputer

The LINC is a 12-bit, 2048-word transistorized computer. The LINC is considered by some the first minicomputer and a forerunner to the personal computer. Originally named the "Linc", suggesting the project's origins at MIT's Lincoln Laboratory, it was renamed LINC after the project moved from the Lincoln Laboratory. The LINC was designed by Wesley A. Clark and Charles Molnar.

MIX is a hypothetical computer used in Donald Knuth's monograph, The Art of Computer Programming (TAOCP). MIX's model number is 1009, which was derived by combining the model numbers and names of several contemporaneous, commercial machines deemed significant by the author. Also, "MIX" read as a Roman numeral is 1009.

PIC microcontrollers series of microprocessors

PIC is a family of microcontrollers made by Microchip Technology, derived from the PIC1650 originally developed by General Instrument's Microelectronics Division. The name PIC initially referred to Peripheral Interface Controller, and is currently expanded as Programmable Intelligent Computer. The first parts of the family were available in 1976; by 2013 the company had shipped more than twelve billion individual parts, used in a wide variety of embedded systems.

Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O processors, commonly known as channels on mainframe computers, which execute their own instructions.

Bank switching

Bank switching is a technique used in computer design to increase the amount of usable memory beyond the amount directly addressable by the processor. It can be used to configure a system differently at different times; for example, a ROM required to start a system from diskette could be switched out when no longer needed. In video game systems, bank switching allowed larger games to be developed for play on existing consoles.

Core rope memory read-only memory in which ferrite cores in a rope act merely as transformers, and whether a word line wire couples or not to the core encodes bits; first used in the 1960s by NASA in Mariner probes and in the Apollo Guidance Computer

Core rope memory is a form of read-only memory (ROM) for computers, first used in the 1960s by early NASA Mars space probes and then in the Apollo Guidance Computer (AGC) designed and programmed by the Massachusetts Institute of Technology (MIT) Instrumentation Lab and built by Raytheon.

Apollo PGNCS Apollo spacecraft guidance system

The Apollo primary guidance, navigation, and control system (PGNCS) was a self-contained inertial guidance system that allowed Apollo spacecraft to carry out their missions when communications with Earth were interrupted, either as expected, when the spacecraft were behind the Moon, or in case of a communications failure. The Apollo command module (CM) and lunar module (LM), were each equipped with a version of PGNCS. PGNCS, and specifically its computer, were also the command center for all system inputs from the LM, including the Kollsman Instrument built alinement optical telescope, the radar system, the manual translation and rotation device inputs by the astronauts as well as other inputs from the LM systems.


HRS-100, ХРС-100, GVS-100 or ГВС-100, was a third generation hybrid computer developed by Mihajlo Pupin Institute and engineers from USSR in the period from 1968. to 1971. Three systems HRS-100 were deployed in Academy of Sciences of USSR in Moscow and Novosibirsk ( Akademgorodok) in 1971 and 1978. More production was contemplated for use in Czechoslovakia and German Democratic Republic (DDR), but that was not realised.

Saturn Launch Vehicle Digital Computer computer of the Saturn V rocket

The Saturn Launch Vehicle Digital Computer (LVDC) was a computer that provided the autopilot for the Saturn V rocket from launch to Earth orbit insertion. Designed and manufactured by IBM's Electronics Systems Center in Owego, N.Y., it was one of the major components of the Instrument Unit, fitted to the S-IVB stage of the Saturn V and Saturn IB rockets. The LVDC also supported pre- and post-launch checkout of the Saturn hardware. It was used in conjunction with the Launch Vehicle Data Adaptor (LVDA) which performed signal conditioning to the sensor inputs to the computer from the launch vehicle.

Jack Garman American computer engineer

John Royer "Jack" Garman was a computer engineer, former senior NASA executive and noted key figure of the Apollo 11 lunar landing. As a young specialist on duty during the final descent stage on 20 July 1969 he dealt with a series of computer alarms which could have caused the mission to be aborted.

When writing firmware for an embedded system, immunity-aware programming refers to programming techniques which improve the tolerance of transient errors in the program counter or other modules of a program that would otherwise lead to failure. Transient errors are typically caused by single event upsets, insufficient power, or by strong electromagnetic signals transmitted by some other "source" device.

Elliott 803

The Elliott 803 is a small, medium-speed transistor digital computer which was manufactured by the British company Elliott Brothers in the 1960s. About 211 were built.

SDS Sigma series series of computers introduced by Scientific Data Systems in 1966

The SDS Sigma series is a series of third generation computers that were introduced by Scientific Data Systems in 1966. The first machines in the series are the 16-bit Sigma 2 and the 32-bit Sigma 7; the Sigma 7 was the first 32-bit computer released by SDS. At the time the only competition for the Sigma 7 was the IBM 360.

Apollo Abort Guidance System

The Apollo Abort Guidance System was a backup computer system providing an abort capability in the event of failure of the Lunar Module's primary guidance system during descent, ascent or rendezvous. As an abort system, it did not support guidance for a lunar landing.

Albert L. Hopkins Jr. is a computer designer. He worked at the US MIT Instrumentation Laboratory during the development of the Apollo Guidance, Navigation, and Control System, or the GN&C. The system was designed in two forms, one for the command module and one for the lunar module. The CM version included an optical system with an integrated scanning telescope and sextant for erecting and correcting the inertial platform. Albert Hopkins received a Ph.D. from Harvard University under Howard Aiken, he then joined the MIT Instrumentation Lab where he was Assistant Director; together with Ramon Alonso, and Hugh Blair-Smith he was a member of the group that designed the computer, designated AGC for Apollo Guidance Computer, identical in the CM and LM.


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Documentation on the AGC and its development
Documentation of AGC hardware design, and particularly the use of the new integrated circuits in place of transistors
Documentation of AGC software operation
Some AGC-based projects and simulators

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